1. Field of the Invention
The present invention relates to a thermal transfer sheet to be used for a thermal transfer printer utilizing a heating means such as a thermal head and a laser, and more specifically to a thermal transfer sheet which provides good printing quality in thermal printing in full color and has an excellent color reproduction property, and is therefore adapted to be used for a poster, an information board, and to a thermal transfer sheet having improved weatherproofing property, wear resistance and chemical resistance property, in which a plastic substrate as an image receiving sheet to which an image has thermally been transferred with the use of the thermal transfer sheet is adapted to be used for an open-air article such as a license plate for a vehicle such as a car, a road sign, or the like.
2. Description of the Related Art
There has conventionally been known a fusion transfer method in which a coloring agent is transferred to a image receiving sheet such as paper and a plastic sheet by impressing energy corresponding to image information by means of a heating device such as a thermal head with the use of a thermal transfer sheet obtained by carrying a coloring layer or a heat fusible coloring ink layer in which coloring agents such as pigment and dye are dispersed in a binder such as heat fusible wax or resin, on a substrate sheet such as a plastic film.
The transferred image formed by this fusion transfer method has high density and is excellent in clarity, and is therefore adapted to record a binary image such as a character or a line drawing. It is also possible to form a polychrome or color image with the use of decreased kinds of color by making a multiple printing record of coloring layers or heat fusible coloring ink layers on an image receiving sheet by using a thermal transfer sheet having the heat fusible coloring ink layers of yellow, magenta, cyan and the like.
However, there have been many conventional thermal transfer sheets having the coloring layer, in which a wax was used as a binder of the coloring layer, and such conventional thermal transfer sheets had a remarkable problem that image-printed material obtained thereby was poor in wear resistance.
In view of this problem, another thermal transfer sheet has been prepared using a resin as the binder of the coloring layer. When a multiple printing of the coloring layer was conducted with the use of such a kind of the thermal transfer sheet, there was however caused a problem of incomplete printing, i.e., a void or a printing defect on an overlapped portion of the coloring layers.
In the conventional thermal transfer sheet having the heat fusible coloring ink layer, the coloring agents used therein, especially pigments of yellow and magenta do not have a high weatherproofing property. As a result, the indoor normal use of the thermal transfer sheet for materials such as a leaflet or a brochure causes no problem, whereas the outdoor use thereof with its exposure to direct sunlight cause a problem of fading.